/Engine/Source/Runtime/Core/Public/Internationalization/ILocalizedTextSource.h
Populate a list of culture names that this localized text source has resource data for (ELocalizationLoadFlags controls which resources should be checked).
void GetLocalizedCultureNames ( const ELocalizationLoadFlags InLoadFlags, [TSet](API\Runtime\Core\Containers\TSet)< [FString](API\Runtime\Core\Containers\FString) > & OutLocalizedCultureNames )